Sanofi's global head of research and development, Elias Zerhouni, said the French drugmaker is working on diagnostic tests to accompany a possible application for U.S. regulatory approval of its Dengvaxia vaccine, sales of which have been stymied by safety concerns.

An analysis conducted after the vaccine's approval found that it should not be given to people who have not already been infected with the virus. On Nov. 29, 2017, Paris-based Sanofi updated the labelling for Dengvaxia, which was first approved in Mexico in 2015 and has been given the go-ahead in 19 countries, to allay safety concerns raised in the Philippines, where dengue fever is endemic, and elsewhere.

Sanofi has reiterated that the vaccine provides persistent protective benefit to those who have previously had the mosquito-borne disease. A diagnostic test could distinguish between those people that have had the virus previously and those that have never been infected, allowing Sanofi to target the specific patient population in whom the vaccine is likely to be effective.

"Dengue is a semi-disappointment to me because for patients who've been exposed, it's a very, very good vaccine," Zerhouni told S&P Global Market Intelligence in an interview. "We're going to continue to make it available and get it approved actually in the U.S. because we believe that for endemic areas there is no other solution."

Sanofi spent two decades developing the Dengvaxia vaccine, which has been through 25 clinical trials in 15 different countries over the last six years. Zerhouni said discussions with the U.S. Food and Drug Administration are ongoing, although he declined to give a specific timeframe for when Sanofi might officially file for approval.

"We want to do an application that shows the benefit/risk ratio of the vaccine," he said. "And basically, direct it primarily to patients who have been exposed to dengue. Now, in the U.S. it may require a specific test, a diagnostic test, which we are working on. Hopefully, we'll get it approved with that indication, not the general indication that we had three years ago when we didn't know about the lack of efficacy in patients who had not been already exposed to the virus."

Dengue fever is a flu-like illness spread by the same female mosquito that transmits the Zika virus and yellow fever. The virus has potentially lethal complications and is a leading cause of serious illness and death in some Asian and Latin American countries, mostly in tropical and sub-tropical regions. The World Health Organization estimates that half of the world's population is now at risk from the virus, with recent estimates of infections put at 390 million people a year.

Reuters reported March 7 that Sanofi was contemplating a U.S. filing for Dengvaxia.